1. Field of the Invention
The present general inventive concept relates to a color filter unit and a projection system employing the same, and more particularly, to a color filter unit capable of preventing a degradation of the quality of an image due to color mixture occurring during combination after separation of light emitted from a white light source, and a projection system employing the color filter unit.
2. Description of the Related Art
Projection systems are classified into 3-panel projection systems or single-panel projection systems according to the number of light valves that are used. A light valve controls the on/off operation of light emitted from a light source (e.g., a high-output lamp) on a pixel-by-pixel basis and forms a picture. Single-panel projection systems can have a smaller optical system than three-panel projection systems but provide only ⅓ of the light efficiency of three-panel projection systems, because red (R), green (G), and blue (B) colors, into which white light is separated, are sequentially used. Hence, attempts to increase the optical efficiency of single-panel projection systems have been made.
Generally, in a single-panel projection system, light radiated from a white light source is separated into R, G, and B colors using a color filter, and the three colors are sequentially sent to a light valve. The light valve appropriately operates according to the sequence of colors received and creates images. As described above, a single-panel optical system sequentially uses colors, so the light efficiency is reduced to ⅓ of the light efficiency of a three-panel optical system. A scrolling method has been proposed to solve this problem. In a color scrolling method, white light is separated into R, G, and B colors, and the three colors are simultaneously sent to different locations on a light valve. Since an image cannot be produced until all of the R, G, and B colors for each pixel reach the light valve, color bars are moved at a constant speed using a specific method, thereby achieving scrolling.
In a conventional single-panel scrolling projection system, as shown in FIG. 1A, white light emitted from a light source 100 passes through first and second lens arrays 102 and 104 and a polarization conversion system 105 and is separated into R, G and B beams by first through fourth dichroic filters 109, 112, 122, and 139. To be more specific, the red beam R and the green beam G, for example, are transmitted by the first dichroic filter 109 and advance along a first light path L1, while the blue beam B is reflected by the first dichroic filter 109 and travels along a second light path L2. The red beam R and the green beam G on the first light path L1 are separated by the second dichroic filter 112. The second dichroic filter 112 transmits the red beam R along the first light path L1 and reflects the green beam G along a third light path L3.
As described above, the light emitted from the light source 100 is separated into the red beam R, the green beam G, and the blue beam B, which are scrolled while passing through corresponding first through third prisms 114, 135, and 142. The first through third prisms 114, 135 and 142 are disposed on the first through third light paths L1, L2, and L3, respectively, and rotate at a uniform speed such that R, G, and B colors are scrolled. A mirror 133 is arranged in the first light path L1. The blue beam B and the green beam G that travel along the second and third light paths L2 and L3, respectively, are transmitted and reflected by the third dichroic filter 139, respectively, and then combined. Finally, the R, G, and B beams are combined by the fourth dichroic filter 122. The combined beam is transmitted by a polarized beam splitter 127 and forms a picture using a light valve 130.
The scrolling of the R, G, and B color bars due to the rotation of the first through third prisms 114, 135, and 142 is shown in FIG. 1B. Scrolling represents the movement of color bars formed on the surface of the light valve 130 when prisms corresponding to the R, G, and B colors are synchronously rotated. A frame is formed when the R, G, and B color bars on the light valve 130 circulate one cycle.
The light valve 130 processes picture information depending on an on-off signal for each pixel and forms a picture. The formed picture is magnified by a projecting lens (not shown) and is projected onto a screen.
To produce a color picture, a projection system having such a structure must perform a process of separating light emitted from the light source 100 into a plurality of color beams and recombining the separated beams.
During such color separation and color combination, a reflectance spectrum of each color beam obtained by a color filter moves toward a short wavelength with a variation of an angle at which the color beam is incident upon the color filter. Hence, color mixture occurs, consequently deteriorating the quality of an image.
The light emitted from the light source 100 is converted into light with an S polarization by the polarization conversion system 105. The light with the S polarization is separated into R, G, and B beams by the first, second, third, and fourth dichroic filters 109, 112, 122, and 139. Variations of spectral reflectivities of the S-polarization R, G, and B beams according to a wavelength are illustrated in FIGS. 2A, 2B, and 2C. FIG. 2A illustrates the spectral reflectivities of rays of the R beam that have different angles, FIG. 2B illustrates the spectral reflectivities of rays of the G beam that have different angles, and FIG. 2C illustrates the spectral reflectivities of rays of the B beam that have different angles. It can be seen from FIGS. 2A through 2C that a half power frequency moves toward a short wavelength with a variation of an incidence angle of a beam.
FIGS. 2A through 2C refer to a case where a beam in the air is incident upon a dichroic filter without passing through any medium. However, FIG. 2D illustrates variations of spectral reflectivities of rays of a red beam that have different angles, when the red beam is incident upon a color filter via a medium. The degree to which a half power frequency moves toward a short wavelength with a change of an incidence angle of an incident beam is greater when the beam was incident upon the color filter via the medium than when the beam was incident upon the color filter via the air.
While a spectral band of each color beam is moving toward the short wavelength with a change in the incidence angle, undesired beams are mixed with desired beams, consequently deteriorating the quality of an image.